1. Field of the Invention
This invention relates generally to methods for reducing the intensity of reflected rays experienced during the exposure process in semiconductor manufacturing. More particularly, the invention relates to methods of utilizing a dielectric anti-reflective coating (DARC) layer to effectively control the intensity of reflected rays encountered during the photolithography process.
2. Background of the Invention
During the exposure process in photolithography, rays that penetrate the photo mask will be transmitted through the surface of the underlying layer and will be reflected by the substrate. These reflected rays will then expose portions of the photoresist that were supposed to be blocked by the mask during the initial exposure. In other words, the reflected rays will result in unwanted exposure in portions of the photoresist causing chemical reactions in the photoresist and the subsequent removal of the photoresist when developed (unwanted photoresist removal will occur for positive photoresist, and vice versa for negative photoresist). The resulting pattern distortion during the exposure process, due to the reflected rays, is referred to as the reflected ray effect.
With the continual shrinkage of line width in semiconductor processing, the proportion of pattern distortion in relation to the overall line width is increasing and will severely effect yield and reliability, causing unwanted shorts and a deterioration in CD (critical dimension) control. To overcome this problem, anti-reflective layer, such as the TiN (titanium nitride) in metal layers, is commonly used to control the intensity of reflected rays.
In more sophisticated technology, it is common practice to use a DARC layer to reduce the amount of reflected rays from non-metalic layers. Due to its ability to absorb rays and to produce a phase shift in the reflected rays, the DARC layer is extensively used in 0.25 .mu.m technology to inhibit the effects of non-metal layer reflection in the photolithography process.
Referring to FIG. 1, the utilization of a DARC layer in the photolithography process is illustrated. It includes a substrate 10, an underlying layer 11 that do not have physical contact with the photoresist, but need to be patterned, and a hard mask layer 12 namely SiO.sub.2 that serves as an indirect photo mask. In the situation in which the photoresist cannot have physical contact with the underlying layer, and in which the underlying layer needs to be defined, a SiO.sub.2 layer would first be deposited as the hard mask 12. The pattern on the photoresist will first be transferred onto the hard mask and the defined hard mask will then be used as an indirect mask to define the underlying layer 11.
Above the hard mask layer 12 are a DARC layer 13 and a photoresist 14. During exposure, the DUV ray 20 enters from the top. Its first reflected ray 21 is the reflection from the DARC layer 13; the second reflected ray 22 is the reflection from the substrate 10, which has already undergone the absorption and the phases shifting when penetrating the DARC layer 13. Hence, when the second reflected ray 22 penetrates the DARC layer 13 and combines with the first reflected ray 21, the phase shifting cancellation occurs between both reflected rays 21 and 22, thereby reducing the intensity of reflected rays.
Theoretically, when a ray penetrates a multi-layer structure, it will produce a reflected ray and a transmitted ray at each interface as shown in FIG. 1. Hence, in theory, there would be a third reflected ray 23 and a fourth reflected ray 24 besides the reflected rays 21 and 22. However, since the DUV ray can easily penetrate the various layers shown in FIG. 1, the third reflected ray and the fourth reflected ray 24 can be combined with the second reflected ray 22.
Nonetheless, the method of suppressing reflected rays during the photolithography process as mentioned above has the following problems:
1) If the hard mask film thickness is non-uniform, it will produce non-uniform reflections. It is not possible to suppress this kind of non-uniform reflections by traditional methods.
2) The DUV active photoresist on the DARC layer will undergo chemical reaction when it has the physical contact with the DARC layer producing a thin amino group (NH.sub.2) film. This will result in the footing effect with the subsequent patterning of the photoresist.
3) During the formation of the DARC layer, the subsequent rapid thermal processing associated with the plasma enhanced chemical vapor deposition (PECVD) process will cause the hard mask layer to undergo physical change as a result of ionic and thermal stress, affecting the etching process of the hard mask that follows.
4) With the DARC layer being on top of the hard mask, the pattern definition process requires the removal of the DARC and hard mask layers. During the etching process of the hard mask layer, the sidewall is easily etched away, forming a bowing profile.